Such a method is known from the French Patent Specification FR-1,567,289. In said method, a polyethylene film having a melt flow index of not more than 0.15 is wound helically around a mandrel, the consecutive windings partially overlapping one another, followed by the provision of an elastomeric layer containing a vulcanizing agent and a sheath which tightly encloses this entity, after which the mutually overlapping parts of the polyethylene windings are fused together and the polyethylene layer and the elastomeric layer are bonded to one another at the same time as the vulcanization of the elastomeric layer. The said melt flow index is determined in accordance with ASTM standard 1238-57 T, with an additional weight of 3 kg being added. A melt index of 0.15 determined in this manner corresponds to a weight-average molecular weight M.sub.w of approximately 0.5.times.10.sup.6 kg/kmol.
A disadvantage of this known process is that said helical winding is less suitable for manufacturing hoses in a simple manner and with adequate production speed in a continous process.
The object of the invention is to provide a process which does not have the said disadvantage.
This object is achieved, according to the invention, in that the polyethylene layer is installed by folding a polyethylene film round in the longitudinal direction in an overlapping manner.
The process according to the invention is found to be extremely suitable for manufacturing, in a continuous process, a hose, clad at least on the inside or outside with a polyethylene layer, in large uninterrupted lengths and with a production speed which is approximately 3-10 times greater than is possible with the known process.
A further advantage of using this method of installing the polyethylene film is that the total surface area of the overlapping seams in the polyethylene cladding layer, in particular if the ratio between the width of the overlap and the diameter of the hose is relatively large, that is to say, especially in the case of hoses having a relatively small diameter, is significantly smaller than in the case of the known process for installing the polyethylene layer, and this commensurately decreases the risk of defects occurring in fusing the seams together and, in addition, results in an appreciable material saving.
It is also an advantage of the process according to the invention that the conventional equipment for manufacturing hoses without a cladding layer can be used as it is or with only minor adaptations.
The process according to the invention can also be used for installing a polyethylene cladding layer on the outside of a hose. In that case, the elastomeric layer is first installed around the mandrel, for example by extrusion, and then the polyethylene film is folded round it. As is usual in vulcanizing rubber hoses, a sheath is then installed around the entity.
By installing the various layers in the desired sequence around the mandrel in one and the same or in consecutive process steps it is also possible to manufacture a hose provided with a polyethylene cladding layer both on the inside and on the outside.
In addition, the process according to the invention can be used to manufacture a hose which contains more than one identical, and if the application requires it, also more than one different, elastomeric layer, if desired, alternating with other layers frequently used in hoses, such as, for example, armouring layers and barrier layers of materials suitable for the purpose, provided only that the polyethylene cladding layer is installed directly adjacently to an elastomeric layer.
A further advantage of the method according to the invention is that it can be used with equally beneficial results starting from already vulcanized hoses. In that case, it is possible to dispense with the use of a mandrel for installing a polyethylene cladding layer on the outside of the hose if desired, because the vulcanized hose in general already has per se adequate stability of shape.